Ground detector



March 18,1969 H,D, KAUFFMAN Y 3,433,918

GROUND DETECTOR I Filed Sept. 2, 1966 49 IE IIOV.A.C.

INVENTOR. 60 6! HARRY D. KAUFFMAN ATTORNEYS United States Patent 3,433,918 GROUND DETECTOR Harry D. Kauifman, Cincinnati, Ohio, assignor to Park- Ohio Industries, Inc., a corporation of Ohio Filed Sept. 2, 1966, Ser. No. 577,053 US. Cl. 21910.77 5 Claims Int. Cl. H0511 1/02 ABSTRACT OF THE DISCLOSURE A detector for indicating the occurrence of a ground on a radio frequency powered induction heating coil includes a capacitor for maintaining the center tap of the coil at ground potential for alternating currents while allowing the coil to remain above ground potential for direct currents and a detector circuit to sense the occurrence of a ground potential on the coil. Radio frequency chokes isolate the radio frequency in the induction heating coil from the detector circuit.

The invention relates to a ground detecting unit for detecting the physical contact of a metallic workpiece with an induction heating coil.

In an induction heat treating apparatus, it is common to load a metallic workpiece which is to be heat treated on a pair of supporting members and transfer the workpiece into the center of a set of induction heating coils. Current is then applied to these induction heating coils to cause heating of selected portions of the workpiece. The workpiece may then be moved out of the heating portion of the machine, unloaded from the supporting mechanism and transferred to a quenching bath. Another workpiece may then be added on the supporting mechanism and transferred into the heating area where the cycle is repeated.

Engagement of the workpiece with any of the induction coils could, of course, cause considerable damage as the work-piece is moved into the heating zone. In order to protect the induction heating coils and the supporting mechanism from damage should a workpiece be misaligned on the centers of the supporting mechanism, a separate part alignment detecting plate, which has the same internal diameter as the induction heating coils, may be provided immediately adjacent the first induction heating coil. Therefore, if the workpiece is misaligned and engages this part alignment plate, it might also engage the heating coils if allowed to continue into the heating zone. On sensing a short circuit between the plate and the frame of the heat treating apparatus, a detector circuit connected to the part alignment plate could, for example, disconnect power from the support mechanism to insure that the workpiece does not continue into the heating zone and thereby cause damage to either the induction heating coil, the drive mechanism or the workpiece itself.

While the part alignment plate of the prior art is particularly useful if the misalignment of the part occurs prior to entering into the heating zone, it might not protect the coils should the part become misaligned after partially entering the induction heating zone. For example, should the leading edge of the workpiece drop after the workpiece is almost completely within the heating zone, the part alignment plate may not detect this misalignment although the workpiece would actually collide with one of the heating coils. Furthermore, the addition of the part alignment plate requires an insulated mounting assembly within the induction heating zone. In order to adequately protect all of the heating coils with the prior art type alignment plates, it would therefore be necessary to include one or more of such plates within the induction heating zone. Congestion of parts within this zone may be eliminated while maintaining adequate protection provided for the induction 3,433,918 Patented Mar. 18, 1969 ice heating coils by the present invention which senses the contact of the workpiece with any of the induction heating coils in the heating zone. 4

The present invention therefore contemplates using the induction heating coils themselves as sensors to detect misalignment of the workpiece. Any contact of the workpiece with any of the induction heating coils may be made to disconnect immediately the power to both the driving mechanism, which transfers the workpiece from the loading zone into the heating zone, and the power to the induction heating coils.

Accordingly, it is an object of this invention to provide an apparatus for use with an induction heating device which will detect the presence of a workpiece in actual contact with the induction heating coils and which may be adapted to terminate the operation of the heat treating apparatus to avoid damage thereto.

It is another object of this invention to provide an apparatus of the type described which requires no additional alignment detecting elements within the heating zone to sense contact of the workpiece with any heating coil and to discontinue machine operations in the event of workpiece alignment.

It is another object of this invention to provide a detecting apparatus of the type described which will detect the actual engagement of the workpiece with any one of the induction heating coils in the heating zone and yet requires no modification of any of the elements within the heating zone.

It is another object of this invention to provide a detecting apparatus of the type described which is of simple construction and which allows close placement of the induction heating coils while providing positive protection against workpiece misalignment throughout the length of the heating zone.

These and other objects and advantages of the invention will be apparent from the following description, the accompanying drawing and the appended claims.

In the drawing:

FIG. 1 is a plan view of a typical induction heating apparatus utilizing this invention; and I FIG. 2 is a schematic electrical diagram showing th ground detector circuit of this invention.

Referring now to the drawings, and particularly to FIG. 1, a heat treating apparatus which might utilize this invention, shown generally at 10, includes a loading hopper 12 containing the workpieces 13 which are to be heat treated. As one workpiece is removed from the hopper by the workpiece supporting mechanism, another workpiece will be moved into position for loading. The workpiece supporting mechanism includes a pair of centers 14 and 15 mounted on carriages 16 and 17 which move the workpiece from a loading position, as shown, to a heating position by a suitable drive means 18. The center 14 is movably mounted with respect to the carriage 16 so that the workpiece may be positioned between the centers when the carriage 16 is in the position shown. The right hand center 15 is attached to the end of a shaft 19 which in turn is secured to the right hand carriage 17. The shaft 19 extends through the induction heating coils 20 and cooling rings 21.

The induction heating coil 20 is mounted in insulative relation to the frame 25 of the heat treating apparatus 10 and is connected to the secondary winding of a high frequency power transformer. The primary winding of this transformer is connected to a suitable power source, such as a high power radio frequency generator, or a lower frequency motor generator source. While the explanation of this invention will be explained in connection with the high or radio frequency type of power source, it is to be understood that either type of power source may be utilized without departing from the scope of this invention.

Once the workpiece is supported between centers 14 and 15, carriages 16 and 17 are moved to the right and into the heating position by a suitable drive means 18, such as a hydraulic cylinder or motor driven lead screw. The workpiece is therefore positioned in the heating zone 27, which includes the zone occupied by the induction heating coil 20 and the cooling rings 21. It would be desirable, when heat treating a cam shaft for example, that the cam shaft lobe be accurately centered within the heating coil, while the shaft area on either side of the cam shaft lobe is positioned within the cooling rings 21. It is apparent, of course, that if the workpiece 13 Where misaligned prior to being transferred into the heating zone, the workpiece could engage the heating coil 20, thus causing either damage to that coil, to the workpiece, or the workpiece support mechanism.

In order to stop the movement of the workpiece in the event of such misalignment, the detection circuit of this invention electrically senses the physical contact of the workpiece with any one of the heating coils by measuring the occurrence of a ground potential applied through the workpiece to the induction heating coil.

Referring now to FIG. 2, which shows a preferred embodiment of the ground detecting circuit of this invention, an induction heating coil 20 is connected to the secondary winding 32 of a radio frequency transformer. It is understood that the coil 20 may either be a single induction heating coil or a plurality of coils connected either in series or in parallel. The primary winding 33 of this transformer is connected to a source of high power radio frequency energy. The particular manner in which this energy is generated is well known in the art and forms no part of this invention. While a radio frequency type transformer is shown and described in this application, it is understood that the power to the induction coil could be supplied by a motor generator source with appropriate modifications in circuit component values.

In order to detect the presence of a workpiece in contact with the induction heating coil 20, a relay circuit is provided which detects the occurrence of a direct current ground potential on any part of the coil 20. For this purpose, a lead 35 is attached at the center portion of the secondary winding 32 of the transformer and maintained at radio frequency ground by capacitor 36. It has been found that for an oscillator frequency of 455 kc., a mfd. capacitor is suflicient to place the center tap at an alternating current ground potential while allowing the coil to remain above ground potential for direct currents.

The addition of lead 35 and capacitor 36 at the center of the secondary winding 32 does not interfere in any way with the operation of the induction heating circuit since the center tap of the secondary is normally at essentially a zero radio frequency potential. A pair of radio frequency chokes 38 and 39 further isolates any radio frequency present on the lead 35 from the remainder of the detecting circuit by presenting a high impedance to any radio frequency current while permitting a direct current path to the induction coil to be maintained.

A two conductor shielded cable 41 interconnects that portion of the circuit thus described which is in close proximity with the induction heating coil and the remainder of the detecting circuit which may be located at a remote position. One conductor 42 in this cable is connected to the shield and to ground potential at each end. The other conductor 43 connects the choke 39 with the relay control circuit 45. This control circuit, in the preferred embodiment, is a commercially available transistor controlled relay manufactured by Cutler Hammer, catalog #13535H98. A capacitor 46 further shorts to ground any radio frequency voltages which may have passed through the radio frequency chokes 38 and 39. Ground conductor 42 is also connected to a portion of the relay circuit 45.

The relay or detector circuit 45 may be used in either of two ways. If it is desired to have a very high sensitivity 4 to voltage change, line 43- may be connected directly to the base electrode of transistor 48. On the other hand, where lower sensitivity is desired, which is the case in most applications since false signals induced on line 43 may trigger the relay circuit and slow down the normal operation of the heat treating equipment, the line 43 would be connected to the emitter electrode of transistor 48, as shown in FIG. 2.

When using the lower sensitivity circuit arrangement, a resistor 49 would be connected between the base electrode and ground. Power for the relay circuit is supplied from a volt alternating current power line through transformer 50 where it is rectified by diode 51. A capacitor 52 aids in smoothing the pulsating direct current output. A voltage divider network including resistors 53 and 54 is connected across the output of the power supply and the junction between resistors 53 and 54 is connected to ground. The collector electrode of transistor 48 is connected to the negative side of the power supply through a coil of relay 56. A diode :57 shunts the coil to dissipate any energy that may be induced into the circuit when current is suddenly removed from this coil.

As long as no workpiece is in contact with the induction coil 20 and shorts it to ground, line 43 and the emitter electrode of transistor 48 will be above ground potential. The base electrode of transistor 48 is normally negatively biased with respect to the emitter electrode, through resistor 59, thus causing the transistor to conduct and thereby energize relay coil 56.

At any time a portion of the workpiece comes in contact with the induction heating coil 20, a circuit will be completed between the frame 25, which is at ground potential, and the induction heating coil 20 through the workpiece 13. A direct current ground potential will then exist on lead 35 and line 43 and cause the emitter electrode to assume a ground potential, cutting off transistor 48 and deenergizing relay 56. The contacts 60 and 61 of relay 56 are in the associated control circuitry for the heat treating apparatus and may be used to disengage the workpiece driving mechanism and to remove power from the induction heating coil at this time.

While the invention has been described for use with a high frequency power source, it is understood that a low frequency supply may also be used provided a few modifications are made in the circuit component values. With a low frequency supply, several induction heating coils 20 may be employed to heat a workpiece with the ground detector attached to one connection of the coils 20 rather than to the center tap of the power supply transformer, as described above. Since low frequencies are used, capacitors 36 and 46 would have a considerably larger value than for the corresponding capacitors in the high frequency embodiment while inductors 38 and 39 would be provided with iron cores.

The apparatus described therefore detects any contact of the workpiece with the induction heating coils without requiring the modification of existing coils or the addition of new components within the heating zone. Furthermore, each coil in the heating zone may be protected from damage since each coil acts as a part alignment detector.

While the form of apparatus herein described constitutes a preferred embodiment of the invention, it is to be understood that the invention is not limited to this precise form of apparatus, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.

What is claimed is:

1. In an apparatus for sensing the misalignment of a workpiece as it passes through an induction heating coil including an induction heating coil and means for supplying power to said coil, the improvement comprising:

a detection circuit connected to said coil for sensing the occurrence of a direct current ground potential on said induction heating coil,

means connected to said coil for maintaining a portion of said coil at ground potential for alternating currents while at the same time allowing the coil to remain above ground potential for direct currents, and

relay means operable by said detection circuit when a workpiece engages said heating coil and causes a direct current ground potential to be applied thereto.

2. The apparatus as defined in claim 1 wherein said means for supplying power to said induction heating coil includes a transformer having a center tap connection on its secondary winding and wherein said detection circuit is connected to said center tap to sense the occurrence of a direct current ground potential thereon.

3. The apparatus as defined in claim 1 further including means between said detection circuit and said induction heating coil for presenting a high impedance to any currents at the power supply frequency to isolate electrically the detecting means from the means supplying power to said coil.

4. The apparatus as defined in claim 1 wherein said detection circuit includes a transistor having one electrode connected to said induction heating coil, means for applying a bias voltage to said transistor to maintain said transistor normally in conduction, and wherein said relay means is included in the collector circuit of said transistor whereby engagement of the workpiece with the induction heating coil will apply a direct current ground potential to said transistor and actuate said relay means.

5. The apparatus as defined in claim 1 wherein said detection circuit includes a transistor having its emitter electrode connected to said induction heating coil, means for applying a bias voltage to said emitter electrode to maintain the transistor normally in conduction, and wherein said relay means includes a normally energized relay in the collector circuit of said transistor whereby engagement of the workpiece with the induction heating coil will apply a direct current ground potential to the emitter electrode to cause said transistor to stop conduction and thereby deenergize said relay means.

References Cited UNITED STATES PATENTS 2,866,067 12/1958 Dolan et a1. 219-519 2,870,310 1/1959 Van Overbeek 21910.77 3,086,999 4/1963 Kramer 219-519 3,376,476 4/1968 Porterfield et a1. 219-10.77 X

RICHARD M. WOOD, Primary Examiner.

L. H. BENDER, Assistant Examiner.

US. Cl. X.R. 317-48 

